Ordinary hydraulic excavators are equipped with a variable displacement hydraulic pump driven by an engine and are formed such that pressure oil discharged from this hydraulic pump is supplied to and drained from various hydraulic actuators through control valves, thereby respectively controlling driving of a work implement, a swivel and a travel gear unit. In such hydraulic excavators, iso-horsepower control for constantly controlling the absorption horsepower [=P (discharge pressure)×Q (discharge flow rate)] of the hydraulic pump is performed in order to match the output torque characteristic of the engine with the absorption torque characteristic of the hydraulic pump in the high fuel efficiency region of the engine.
There is a known technique (e.g., Patent Document 1) according to which a hydraulic excavator of the above type is provided with (i) a main reflux flow path for flowing hydraulic oil back to a tank through a control valve, the hydraulic oil being pushed out from an arm cylinder during arm dumping operation in which the arm is operated to turn in a forward direction and (ii) a sub reflux flow path for directly flowing part of the hydraulic oil back to the tank. With this arrangement, pressure loses in the return circuit during the arm dumping operation can be restricted thereby lowering working pressure to reduce the loss of oil pressure.
Patent Document 1: Japanese Unexamined Published Patent Application No. 2002-339904
There is another known technique according to which two hydraulic pumps of the above type are serially disposed and switching between a split-flow condition and an interflow condition is enabled. In the split-flow condition, the discharge oil of one of the hydraulic pumps is supplied to the arm cylinder whereas the discharge oil of the other is supplied to the bucket cylinder. In the interflow condition, the discharge oils of the hydraulic pumps are joined together and supplied to either one of the arm cylinder and the bucket cylinder in preference to the other. In the split-flow condition, the loss of oil pressure can be reduced. In the interflow condition, the excavating operation of either the arm or bucket can be speeded up.